I'd like to know if there is a particular reason to ban free e-mail adresses? I'd like to join with the mail account I most frequently use, which is not a paid e-mail (gmail). I've given up on joining but I'd like to know why?

the main reason is with multiple accounts. if we allowed free email addresses to sign up, tracking down multi-accounts would be much harder, since everyone could sign up thousands of times with free email addresses. restricting signup to non-free email addresses greatly limits the possibility of creating multi-accounts.